In a steering apparatus for a vehicle, if a driver rotates a steering wheel in a desired direction, a steering shaft connected to the steering wheel rotates. Accordingly, the steering shaft delivers rotational force to a gear box including a rack-pinion gear through a universal joint and a slip joint.
At this time, the gear box converts the rotational movement of the steering shaft to a linear movement through the rack-pinion gear so as to transfer it to a rack bar. The rack bar transfers force to a tie rod connected to a knuckle of a tire so as to allow change of the drive direction of a vehicle.
Particularly, a shaft positioned between the steering shaft and the gear-box has a structure where input and output shafts are slanted with respect to each other at a predetermined angle, and are not positioned on the same axis. This is because a typical type shaft assembling structure can not transfer power. Therefore, it is necessary to use a universal joint allowing the structure where a steering shaft can be slanted at a predetermined angle.
FIG. 1 is a schematic side view of a steering apparatus using a typical universal joint. Generally, when the lower end of a steering shaft 210 is connected with a gear box 260 in a steering system of a vehicle, it is required for the steering shaft to be connected with the gear box in a state where the steering shaft is slanted at a certain angle in consideration of peripheral conditions. Therefore, the steering system uses a universal joint 100 so as to satisfy such a condition.
Herein, the term ‘the universal joint’ 100 is used as a common term designating a member which includes an outer member 122, the inner member 124, etc. and transfers rotational force even in a state where the member is bent at a predetermined angle.
The universal joint 100 is a device which has one side coupled with a steering shaft 210 assembled with a steering wheel 270 and the other side coupled with a gear box 260 so as to transfer rotational force generated in the steering wheel 270 to wheels of a vehicle through the gear box 260.
A steering column 250 includes the outer tube 230, the inner tube 220 and a mounting bracket 240. The outer tube 230 is formed in a direction of the steering wheel 270, and has the inner tube 220 inserted into the outer tube. The inner tube 220 has a diameter smaller than the diameter of the outer tube 230 so that the inner tube can be inserted within the interior of the outer tube when impact is exerted thereto. The outer tube 230 and the inner tube 220 are hollow and are formed in a tube-shape so as to allow the steering shaft 210 to smoothly rotate.
FIG. 2 is a perspective view of a conventional universal joint of a steering apparatus for a vehicle. FIG. 3 is a sectional view of a conventional slip joint of a steering apparatus for a vehicle.
As shown in FIGS. 2 and 3, the conventional universal joint 100 includes yoke joints 110, which transfer rotational force, even in a state of they are bent at a predetermined angle, and a slip joint 120, which transfers the rotational force transferred from one of the yoke joints 110, which is installed at one side of the slip joint, to the other yoke joint 110 installed at the other side thereof and simultaneously can be extended and contracted in an axial direction.
Each yoke joint 110 includes two yokes 130 and one spider 140. The slip joint 120 allows the shaft to be extended and contracted by force applied in an axial direction while transferring rotational force of the shaft. The slip joint 120 has an outer member 122 having a hollow-shape, an inner member 124 inserted into the outer member 122 so as to absorb force applied in a longitudinal direction, and a ball 150 disposed between the outer member 122 and the inner member 124.
The outer member 122 has a hollow interior, and the inner member 124 having a shape of a circular rod is connected with the outer member in such a manner that it is inserted within the outer member 122. Each guide recess 160 is formed at both sides of the inner circumferential surface of the outer member 122 in such a manner that the guide recess is formed along an axial direction while having a long shape, in order to allow the ball 150 to move together with the inner member 124 when a slip operation where the inner member 124 retracts into or protracts from the outer member 122 is performed after the ball 150 is inserted between the inner member and the outer member.
The inner member 124 has one side connected with the yoke joint 110 and the other side assembled with the outer member 122 so that the inner member transfers rotational force and is simultaneously inserted within the outer member 122, thereby absorbing load applied in a longitudinal direction between an input shaft and an output shaft while slipping in an axial direction.
Each guide recess 170 is formed at a position of the outer circumferential surface of the inner member 124, which corresponds to the guide recess 160, in a longitudinal direction so as to allow the ball 150 inserted with the guide recess 160 to stably and smoothly move.
The ball 150 is inserted between the outer member 122 and the inner member 124 in such a manner that the ball makes close contact with the guide recess 160 and the guide recess 170, and slips under rolling friction. At this time, as the inner member 124 rotates, the outer member 122 rotates together with the inner member so that rotational force from the input shaft is transferred to the output shaft.
However, the conventional universal joint 100 of a steering apparatus for a vehicle has a problem in that when bending force or torsion force generated due to large load exerted from the outside is applied to the universal joint 100, the ball 150 disposed between the outer member 122 and the inner member 124 rotates while receiving the load. As a result, a deformation phenomenon where the ball 150 is damaged or deteriorated occurs so that the ball can not smoothly transfer rotational force of the input shaft. Also, because the ball disposed between the guide recess 160 and the guide recess 170 is worn away due to frequent friction, etc., clearance is generated so that a rattling noise is generated.
In addition, in the conventional universal joint 100 of a steering apparatus for a vehicle, the guide recess 160 and the guide recess 170 have a semi-circular section where a radius of curvature is equal in whole part so that each surface of the guide recess 160 and the guide recess 170 makes linear contact with the spherical surface of the ball 150, thereby generating larger frictional resistance. Because of deformation of the outer member 122 or the inner member 124, which is generated because errors occur in manufacturing measurement, or the straightness degree is changed, there is a problem in that smooth rotation of the ball 150 is obstructed so that a sliding performance is changed.